This invention relates to a power converter apparatus and, more particularly, to improvements in a power converter apparatus using a controllable switching means having a current cut-off function.
Heretofore various methods for AC--DC power converter apparatus have been proposed and put into practical use. Particularly, a converter apparatus in which alternating current is converted into direct current by means of phase control using a thyristor or the like has been widely used.
Additionally, a power converter apparatus using, in place of a thyristor, a gate turn-off thyristor (hereafter called "GTO") which is combined with a transistor or the like for a switching element which compulsorily cuts off energized current has been adopted. However, overvoltage is inconveniently generated when this GTO interrupts current.
As a way of solving this problem, a method has been proposed for protecting the GTO by connecting a filter condenser to the power source side of a bridge circuit, for example, as is shown in FIG. 8 of the "POWER CONVERTER APPARATUS" disclosed in Japanese Patent Laid-Open No. 83588/1983 which was laid open in Japan on May 19, 1983.
In this method, GTO of a small dielectric strength can be used because when the GTO interrupts the energized current I.sub.UGTO, the voltage which is generated at ACL.sub.U and ACL.sub.V is absorbed by filter condensers C.sub.U, C.sub.V and C.sub.W. However, this method has the following defect: In the transition from the reflux period of an electric circuit (2), which consists of a load L composed of a voltage R.sub.L, a reactance L.sub.L and a direct current source E.sub.L --a thyristor V.sub.2 --a diode D.sub.V O a GTO (V.sub.GTO)--and a load L, to the conducting period of a current circuit (1), which consists of a power source U--a U phase reactance AC.sub.LU --a GTO (U.sub.GTO)--a load L--a thyristor V.sub.2 --a V phase reactance ACL.sub.V --and a power source V, discharge current flows from the filter condensers C.sub.U, C.sub.V and C.sub.W to a current U.sub.GTO --V.sub.GTO --D.sub.V --C.sub.U without the medium of the power source reactor and after interrupting the reflux current of the electric circuit (2), it supplies energy to the load L.
Since, when the discharge current flows, there is nothing except the inductance of the wiring in the circuit, voltage of high voltage regulation (dv/dt) is applied to the U.sub.GTO and the thyristor U.sub.2 (hereafter called "switching means"), and the voltage which is beyond the limit value of the voltage regulation of the U.sub.GTO and the U.sub.2 is applied to the switching means, whereby these switching means are disadvantageously damaged.